Re: FAILED: patch "[PATCH] ALSA: timer: Fix mutex deadlock at releasing card" failed to apply to 4.19-stable tree

[Sasha Levin <sashal@kernel.org>]

On Sun, Nov 03, 2019 at 06:40:59PM +0100, gregkh@linuxfoundation.org wrote:
>
>The patch below does not apply to the 4.19-stable tree.
>If someone wants it applied there, or to any other stable or longterm
>tree, then please email the backport, including the original git commit
>id to <stable@vger.kernel.org>.
>
>thanks,
>
>greg k-h
>
>------------------ original commit in Linus's tree ------------------
>
>From a39331867335d4a94b6165e306265c9e24aca073 Mon Sep 17 00:00:00 2001
>From: Takashi Iwai <tiwai@suse.de>
>Date: Wed, 30 Oct 2019 22:42:57 +0100
>Subject: [PATCH] ALSA: timer: Fix mutex deadlock at releasing card
>
>When a card is disconnected while in use, the system waits until all
>opened files are closed then releases the card.  This is done via
>put_device() of the card device in each device release code.
>
>The recently reported mutex deadlock bug happens in this code path;
>snd_timer_close() for the timer device deals with the global
>register_mutex and it calls put_device() there.  When this timer
>device is the last one, the card gets freed and it eventually calls
>snd_timer_free(), which has again the protection with the global
>register_mutex -- boom.
>
>Basically put_device() call itself is race-free, so a relative simple
>workaround is to move this put_device() call out of the mutex.  For
>achieving that, in this patch, snd_timer_close_locked() got a new
>argument to store the card device pointer in return, and each caller
>invokes put_device() with the returned object after the mutex unlock.
>
>Reported-and-tested-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
>Cc: <stable@vger.kernel.org>
>Signed-off-by: Takashi Iwai <tiwai@suse.de>

Looks like this was introduced by 41672c0c24a6 ("ALSA: timer: Simplify
error path in snd_timer_open()"), which means it's not needed on 4.19 or
older.

-- 
Thanks,
Sasha